1. Field of the Invention
The invention relates in general to web tracking devices of the type in which a tensioning and steering roller resiliently applies tension to the web uniformly across its width and is tilted in response to lateral movement of the web to steer the web and thereby maintain it within a predetermined path. More particularly, the invention relates to such a device in which the steering position of the roller is established by roller supporting structure including support means cooperating with ramp means such that the tilt of the roller is effected by relative lateral movement between the ramp means and the support means and in which the same elements of the roller supporting structure that define the steering position of the roller also at least partially support the roller to provide for the roller movement by which the web is uniformly tensioned.
2. Description Relative to the Prior Art
The type of web tracking device to which the present invention is directed is typically used in conjunction with an endless web or belt, for example, the endless photoconductor belt in an electrostatographic apparatus such as a copier or printer. In order to tension the belt, to compensate for belt conicity and to steer the belt to maintain it within its desired path, the tensioning and steering roller must have at least three degrees of freedom in addition to its freedom to rotate about its own axis: namely, one degree of translational freedom to allow the roller to move generally parallel to its axis to tension the belt; a first or tilting degree of rotational freedom to provide the required belt steering movement of the roller; and a second or skewing degree of rotational freedom to allow the roller to maintain equal tension across the width of the belt, thereby compensating for unavoidable inaccuracies resulting in belt conicity.
U.S. Pat. No. 4,893,740, issued in the name of Edwin A. Hediger et al. to the assignee of the present invention on Jan. 16, 1990, discloses a web tracking device of the general type described above, in which the steering roller is rotatably carried by a yoke that is pivotable about a steering or gimbal axis defined by a support rod rotatably and slidably mounted in a housing that, in turn, is pivotable about a caster axis. The support rod is spring loaded to urge the yoke away from the housing to tension the web and the movement of the entire assembly about the caster axis provides for uniform tension across the width of the web by allowing the roller to assume an angular position in which it can compensate for belt conicity.
The term "belt conicity" literally means that an endless belt is slightly conical, rather than being perfectly cylindrical, but, as used herein, the term means either that an endless belt or web is slightly conical or that a belt or web, whether endless of not, behaves as if it were endless and slightly conical, either because of inaccuracies in the belt or web per se or in other parts of the belt or web guiding or supporting mechanism. Although the term "web" is generally perceived as being more generic than the term "belt" and the latter sometimes implies endlessness, the two terms, as used herein, should be considered as interchangeable.
In the Hediger device, flanges at both ends of the steering and tensioning roller are axially movable with respect to that roller. When the web drifts laterally in either direction, it engages and displaces the flange at the corresponding end of the roller, and this, in turn, shifts a slidably movable yoke driver in the same direction. The yoke driver includes oppositely sloped slots engaged with corresponding pins extending from the yoke, whereby the lateral movement of the yoke driver causes the roller to tilt in the proper direction to counteract the drifting tendency of the web.